Galaxy Merger

Galaxy mergers can occur when two or more galaxies collide. They are the most violent type of galaxy interaction. Gravitational interactions between galaxies and friction between gas and dust have profound effects on the involved galaxies. However, the precise effects of such mergers depend on a wide range of parameters including collision angles, velocities, and relative size/composition like, and remain an active area of research at the forefront of degree research.

Galaxy mergers are important because the merger rate is a fundamental measure of galaxy evolution. They also provide astronomers with clues about how galaxies grew into their current forms across different cosmic epochs long time.

During a merger, stars and darkness substance in each galaxy are affected by the approaching galaxy. As the merger approaches its final stages, the gravitational potential changes so dramatically that stellar orbits are significantly altered and lose all memory of their previous paths. This process is called “violent relaxation.” For example, when two disk galaxies collide, stars that previously orbited in two distinct planes begin to move in random orientations. During the merger, this ordered motion is converted into random kinetic energy and thermalized. In the resulting galaxy, stars orbit in a complex and randomly interacting network of paths, as observed in elliptic galaxies.

Mergers are also sites of extreme star formation. During a major merger, the star formation rate (SFR) can reach thousands of sun solar masses per year, depending on the gas content and redshift of each galaxy. Typical merger SFRs are less than 100 new solar masses per year. This is significantly higher than the Milky Way Galaxy, which produces only a few new stars annually (approximately two). Although stars in galaxy mergers rarely come close enough to collide, work molecular clouds rapidly fall toward the galactic center, where they collide with other molecular clouds. These collisions trigger the collapse of the clouds into new stars. However, this process was more prominent during the mergers that formed most of the elliptical galaxies observed today, which likely occurred 1–10 billion years ago when galaxies contained far more gas and molecular clouds. Additionally, away from the galactic center, gas clouds colliding with each other generate shocks that promote new star formation within the clouds. As a result of all this activity, galaxies tend to have very little Gaza left after merging. Therefore, if a galaxy undergoes a major merger and several billion years pass, very few young stars will remain in the galaxy.

This is precisely what is observed in elliptical galaxies today: very little molecular gas and very few young stars. It is believed that elliptical galaxies are the end products of major mergers that consumed most of their gas, thereby quenching further star formation after the merger.

^{Two galaxies in the process of merging. (Credit:} NASA, ESA and the Hubble Team⁾

^{The galaxy NGC 3256, located about 100 million light years away, formed from the merger of two galaxies. (Credit:} NASA, ESA and the Hubble Team⁾

^{The Taffy Galaxies collision. (Credit:} International Gemini Observatory⁾

Galaxy mergers are classified into different categories based on characteristics such as the number of merging galaxies, their relative sizes, and their gas richness.

Mergers can be categorized by the number of galaxies involved:

• Binary Merger: The merger of two galaxies in Interaction.
• Multiple Merger: The merger of three or more galaxies.

Mergers can be classified based on how much the largest galaxy involved changes in size or shape during the merger.

• Minor Merger: A merger is considered minor if one galaxy is significantly larger than the other. The larger galaxy typically “place” the smaller galaxy. This process, known as “galactic cannibalism,” has minimal other significant effects on the larger galaxy, as it primarily absorbs the gas and stars of the smaller one. It is thought that the Milky Way Galaxy, including World, is currently in the process of consuming several smaller galaxies in this way, such as the Canis Major Dwarf Galaxy and possibly the Magellanic Clouds.
• Major Merger: The merger of two spiral galaxies of approximately equal size is a major event. If they collide at suitable angles and velocities, they will likely merge in a way that ejects much of their dust and gas, often through feedback mechanisms associated with active galactic nuclei. This is thought to be the driving force behind many quasars. The result is an elliptical galaxy, and many astronomers consider this the primary mechanism for forming elliptical galaxies.

Mergers can be categorized based on the degree of interaction between the gas within and around the merging galaxies.

• Wet Merger: A wet merger occurs between gas-rich galaxies (blue galaxies). Wet mergers typically produce large amounts of star formation, transform disk galaxies into elliptical galaxies, and trigger quasar activity.
• Dry Merger: A dry merger occurs between gas-poor galaxies (red galaxies). Dry mergers typically do not significantly alter the star formation rates of galaxies but play an important role in increasing stellar mass.
• Damp Merger: A damp merger occurs when there is sufficient gas between blue and red galaxies to fuel significant star formation, but not enough to form globular clusters.
• Mixed Merger: A mixed merger occurs when a gas-rich galaxy merges with a gas-poor galaxy karma.

^{Predicted merger between the Milky Way Galaxy and the Andromeda Galaxy. Image representing Earth’s night sky 3.75 billion years from now. (Credit:} NASA, ESA, Z. Levay, R. van der Marel⁾